The present invention relates to magnetic resonance imaging (MRI) apparatus and more particularly is concerned with a technique known as pre-polarisation by which a low magnetic field may be temporarily increased for the purpose of enhancing the sensitivity of a relatively low field NMR (Nuclear Magnetic Resonance) experiment.
The aforementioned pre-polarisation technique will now be described by way of background information.
This is a technique in which a large low quality magnetic field is temporarily applied to an object to be studied by NMR particularly in an NMR environment where there is a low, or even zero magnetic field. With this technique a large field is first applied for a period to cause the target object to become magnetised to a level related to that field. The magnetisation produced by the high field develops with the time constant T1 characteristic of the high field.
The polarising field is then very rapidly removed so that the target object magnetisation decays back towards that which was applied originally with the value of the time constant T1 which is characteristic of the lower field.
This allows quite a long period (typically less than 0.5 T1 at the low field) during which data can be recovered but during which the magnetisation in the target object is well above what would be expected in the low field. Thus it is possible, in principle, to enhance the sensitivity of a low field NMR experiment quite substantially using this technique as long as the time constants with which the magnetisation decays are long compared with the time taken to remove the pre-polarisation field.
Pre-polarisation has been proposed as a possibility for low field MRI (U.S. Pat. No. 5,296,811) but, as yet, has not been demonstrated in vivo. This is because at all except the lowest fields or with poorly coupled coils, detector noise in human MRI is dominated by noise from the patient's body itself, and most current machines operate at sufficiently high fields for this to be true. It has been pointed out that, even at best, the signal-to-noise ratio is proportional to B.sub.0, the magnitude of the main field. In practice, the gain is less than that because of timing limitations.
The Applicants have however appreciated that one major advantage of pre-polarisation as against attempting to increase the main magnetic field level is that the quality i.e. degree of uniformity of pre-polarising field required is relatively low.
The rapid development of interventional MRI is the principal driver leading to the development of increasingly open magnets, allowing much better access to patients than that allowed by the traditional experimental configuration. Improved accessibility is, however, paid for by the difficulty of achieving high field levels in otherwise useful designs. Pre-polarisation offers the possibility of getting substantially enhanced images from local regions of the body of immediate concern to a clinician performing a procedure.